doc/go1.17: further revise OpenBSD release notes
doc/go1.17: further revise OpenBSD release notes

Simplify and remove forward-compatibility reference, as OpenBSD 6.9 has
already been released (1st of May 2021).

Updates #44513

Change-Id: I0a1abbb397f31d15c80a970edaa9723f894cafa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/327652
Trust: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go